The USRP has a DC offset removal filter in the FPGA. A carrier sent at 0Hz is exactly the same as DC offset, and will be removed. Zero-IF receiver architectures (like the USRP) intrinsically have DC offset imperfections. The filter can be disabled in the USRP Source "FE Options" tab. It is not good practice to design a system which depends on DC in a zero-IF receiver, because the DC offset error will change over frequency, temperature, time, etc.
On Thu, Jul 18, 2019 at 4:29 AM Simona Sibio <ssib...@gmail.com> wrote: > Hi all, > > Thank you very much for your emails. > But no tip works fine. > I wanted only to learn how is USRP working with GNU radio, I thought that > the RX chain in the USRP was different. > But I am not the beginner in electronic and RF circuits. > Anyway, no problem, I don't ask more information about this. > Best Regards, > > Simona > > Il giorno gio 18 lug 2019 alle ore 12:11 Jonas Manthey < > jonas.mant...@u-blox.com> ha scritto: > >> Hi, >> >> >> >> You don’t send a sine with frequency 0, you send a carrier that is >> modulated by a sine with a frequency of 0. A sine with a frequency of 0 is >> 0: sin(2 pi f) = sin(2 pi 0) = sin(0) = 0. >> >> >> >> So your carrier is modulated with a zero, which I **think** ends up in >> no carrier at all, not sure right now how zero-IF architectures behave in >> that case. I highly recommend reading some communications or RF >> book/website/guide or talk with your advisor. >> >> >> >> UHD-USRP clips anything above 1 if left in the default setting (float I >> guess) and there will be a lot of garbage in the sent spectrum. >> >> >> >> Cheers, >> >> Jonas >> >> >> >> *From:* Discuss-gnuradio [mailto:discuss-gnuradio-bounces+jonas.manthey= >> u-blox....@gnu.org] *On Behalf Of *Simona Sibio >> *Sent:* Donnerstag, 18. Juli 2019 13:01 >> *To:* Kyeong Su Shin <kss...@postech.ac.kr> >> *Cc:* discuss-gnuradio@gnu.org >> *Subject:* Re: [Discuss-gnuradio] USRP GNU radio receiver >> >> >> >> Hi Kyeong, >> >> >> >> I attached the screenshots about time-domain and frequency domain. >> >> I sent a sine with frequency=0 and amplitude=0,25. >> >> The output is always zero in the receiver. >> >> The situation changes only if I send a sine with a frequency non-zero. >> >> Thank you for your time. >> >> Best Regards, >> >> >> >> Simona >> >> >> >> Il giorno mer 17 lug 2019 alle ore 17:41 Kyeong Su Shin < >> kss...@postech.ac.kr> ha scritto: >> >> To whom it may concern: >> >> >> >> I think there has been some miscommunications or misunderstandings here: >> the 'Offset' argument for the 'Signal Source' block is for the 'amplitude >> offset' of the signal, while what Marcus probably meant in his previous >> e-mail is the 'frequency offset' (you can use USRP's low-freq digital IF >> capabilities for that). Also, I do not know how UHD sink behaves when you >> do not input normalized values, but I am pretty sure that you should not >> input such large values to UHD sink. >> >> >> >> If readings in the frequency-domain are non-zero (or, in real-world >> devices, extremely low values), readings in the time-domains are also >> non-zero. So, "the output became unstable in frequency and in time is >> always zero" doesn't make sense. Your time-domain plot is probably zoomed >> out too much. >> >> >> >> Finally, as Marcus mentioned, throttle blocks _must_ be removed. >> >> >> >> Regards, >> >> Kyeong Su Shin >> >> >> ------------------------------ >> >> *보낸* *사람**:* Simona Sibio <ssib...@gmail.com> 대신 Discuss-gnuradio >> <discuss-gnuradio-bounces+ksshin=postech.ac...@gnu.org> >> *보낸* *날짜**:* 2019년 7월 17일 수요일 오후 11:35:00 >> *받는* *사람**:* Müller, Marcus (CEL) >> *참조**:* discuss-gnuradio@gnu.org >> *제목**:* Re: [Discuss-gnuradio] USRP GNU radio receiver >> >> >> >> Thank you very much! >> >> I sent the screenshot because in the oscilloscope there is not the >> offset. >> >> Instead, I would like to detect that. >> >> Thank you in advance. >> >> >> >> Simona >> >> >> >> Il giorno mer 17 lug 2019 alle ore 15:22 Müller, Marcus (CEL) < >> muel...@kit.edu> ha scritto: >> >> 1. Never use a throttle with a hardware block. GRC's log will actually >> shout at you that you shouldn't be doing that! >> 2. I must admit this is about as good as I'd expect. I of course can't >> infer temporal behaviour from screenshots. >> >> Best regards, >> Marcus >> >> On Wed, 2019-07-17 at 14:18 +0100, Simona Sibio wrote: >> > Hi Marcus, >> > >> > I tried to change the offset in the transmitter but the output in the >> receiver is the same, also if I send a sine signal with 10 Hz and 0,1 >> amplitude: the output became unstable in frequency and in time is always >> zero (see attached, 130K). >> > The flow graph is attached. >> > Do you have any suggestions? >> > Thank you in advance. >> > >> > Simona >> > >> > Il giorno mar 16 lug 2019 alle ore 18:59 Müller, Marcus (CEL) < >> muel...@kit.edu> ha scritto: >> > > That phase is not any more or less random than that of your local >> > > oscillator, so there might be some misunderstanding here. >> > > On Tue, 2019-07-16 at 18:52 +0100, Simona Sibio wrote: >> > > > Thank you very much! >> > > > But, with the offset tuning I cannot edit the phase. >> > > > Maybe, I have to change method. >> > > > Best Regards, >> > > > >> > > > Simona >> > > > >> > > > Il giorno mar 16 lug 2019 alle ore 18:25 Müller, Marcus (CEL) < >> muel...@kit.edu> ha scritto: >> > > > > Well, there's leakage / DC blocking filters in most mixers, so >> yes. >> > > > > >> > > > > But the solution is simple (and hardware-supported transparently >> on >> > > > > USRPs): Just tune at an offset, and mix down to 0 Hz >> (==constant!) in >> > > > > digital. Your USRP can do that for you, google for "USRP offset >> > > > > tuning". >> > > > > >> > > > > Best regards, >> > > > > Marcus >> > > > > >> > > > > On Tue, 2019-07-16 at 18:15 +0100, Simona Sibio wrote: >> > > > > > Thank you very much!! >> > > > > > Then, I cannot measure a constant because, in the RX chain, >> maybe, some filter deletes the information, doesn't it? >> > > > > > Thank you for your time and tour tips! >> > > > > > >> > > > > > Best Regards, >> > > > > > >> > > > > > Simona >> > > > > > >> > > > > > Il giorno mar 16 lug 2019 alle ore 18:01 Müller, Marcus (CEL) < >> muel...@kit.edu> ha scritto: >> > > > > > > Look up what a quadrature modulator does. It's just a >> multiplication >> > > > > > > with e^{j2\pi f_{carrier}t}, in the end. >> > > > > > > >> > > > > > > Your constant signal s(t) = c const. would hence be >> transmitted as >> > > > > > > >> > > > > > > c·e^{j2\pi f_{carrier}t} >> > > > > > > >> > > > > > > whereas a complex sinusoid of frequency f and amplitude a, >> > > > > > > s(t) = a·e^{j2\pi f t) would be transmitted as >> > > > > > > >> > > > > > > a·e^{j2\pi f t)·e^{j2\pi f_{carrier}t} = a·e^{j2\pi >> (f+f_{carrier})t} >> > > > > > > >> > > > > > > so, identical to the constant signal, just at a different >> frequency. >> > > > > > > >> > > > > > > >> > > > > > > Best regards, >> > > > > > > Marcus >> > > > > > > >> > > > > > > On Tue, 2019-07-16 at 17:53 +0100, Simona Sibio wrote: >> > > > > > > > Hi Marcus, >> > > > > > > > >> > > > > > > > Thank you for tip! >> > > > > > > > I know that calibration is not so easy but I hope that. >> > > > > > > > Could you help me to understand what happened with a >> constant complex baseband signal? >> > > > > > > > I am beginner in this field. >> > > > > > > > Thank you for your time. >> > > > > > > > >> > > > > > > > Best Regards, >> > > > > > > > >> > > > > > > > Simona >> > > > > > > > >> > > > > > > > Il giorno mar 16 lug 2019 alle ore 17:39 Müller, Marcus >> (CEL) <muel...@kit.edu> ha scritto: >> > > > > > > > > Watch out! What do you get at the *output* of your SDR >> device when you >> > > > > > > > > transmit a constant complex baseband signal, mixed to a >> carrier >> > > > > > > > > frequency? How does that differ from transmitting a >> complex baseband >> > > > > > > > > sinusoid mixed to a carrier frequency? >> > > > > > > > > >> > > > > > > > > On Tue, 2019-07-16 at 18:32 +0200, sumit kumar wrote: >> > > > > > > > > > Ok, so for a constant source, in the time domain you >> will see a flat signal only! Try connecting a spectrum analyzer and you >> shud see a peak at DC becz Fourier transform of constant signal is impulse. >> > > > > > > > > > And yes, as Marcus said, increase the sampling rate! >> > > > > > > > > > >> > > > > > > > > > >> > > > > > > > > > On Tue, 16 Jul 2019 at 18:06, Simona Sibio < >> ssib...@gmail.com> wrote: >> > > > > > > > > > > Thank you for the assistance. >> > > > > > > > > > > >> > > > > > > > > > > I put the module "constant source" in my flow grap. >> > > > > > > > > > > With this module, I can choose witch amplitude I want >> to send. >> > > > > > > > > > > I attached the flow graph. >> > > > > > > > > > > And, I would want to read these values in the >> receiver. >> > > > > > > > > > > >> > > > > > > > > > > Simona >> > > > > > > > > > > >> > > > > > > > > > > Il giorno mar 16 lug 2019 alle ore 16:58 sumit kumar < >> sumits...@gmail.com> ha scritto: >> > > > > > > > > > > > Hi Simona, what is the "constant signal" here ? >> > > > > > > > > > > > >> > > > > > > > > > > > On Tue, 16 Jul 2019 at 17:54, Simona Sibio < >> ssib...@gmail.com> wrote: >> > > > > > > > > > > > > Hi all, >> > > > > > > > > > > > > >> > > > > > > > > > > > > I want to use GNU radio to measure the amplitude >> and the phase of a signal. >> > > > > > > > > > > > > I send a constant signal with the transmitter >> USRP but in the receiver there is a flat signal with amplitude 0 and offset >> 0. >> > > > > > > > > > > > > I tried to send a sine signal and the receveir >> works fine. >> > > > > > > > > > > > > How can I do to send only a constant and measure >> the amplitude? >> > > > > > > > > > > > > >> > > > > > > > > > > > > Thank you for your time. >> > > > > > > > > > > > > >> > > > > > > > > > > > > Simona >> > > > > > > > > > > > > >> > > > > > > > > > > > > _______________________________________________ >> > > > > > > > > > > > > Discuss-gnuradio mailing list >> > > > > > > > > > > > > Discuss-gnuradio@gnu.org >> > > > > > > > > > > > > >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> > > > > > > > > > > > >> > > > > > > > > > > > >> > > > > > > > > > > > -- >> > > > > > > > > > > > Sumit Kumar >> > > > > > > > > > > > >> > > > > > > > > > > > >> > > > > > > > > > >> > > > > > > > > > >> > > > > > > > > > _______________________________________________ >> > > > > > > > > > Discuss-gnuradio mailing list >> > > > > > > > > > Discuss-gnuradio@gnu.org >> > > > > > > > > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> >> _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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